1. Field of the Invention
The invention relates generally to external navigation lighting for aircraft and more particularly to an LED aircraft taillight that appears white but emits little or no light in the upper (red) portion of visible light spectrum and the just above visible spectrum (near infra red).
2. Description of the Related Art
Civilian air traffic control agencies such as the FAA specify standards for aircraft external lighting. Aircraft operating at night in civilian airspace are required to display lights to attract the attention of other aircraft operating in the same airspace. These external aircraft lights include flashing anticollision lights mounted on the aircraft's upper and lower fuselage, as well as steady burn position/navigation lights on the tail and the wing tips. The location, color, intensity and light radiation pattern for each particular light is typically specified by the relevant regulation.
Night vision imaging systems (NVIS) were introduced in the late 1960's and are now widely used by military and law enforcement personnel to enhance the effectiveness of aircraft during night operations. Night vision goggles (NVGs) are binocular-looking devices that can be attached to a flight helmet or a combat helmet and suspended in front of the eyes. NVGs are now widely appreciated as one of the best and most cost-effective ways to permit night operations in aircraft.
NVGs operate by greatly multiplying very low levels of existing light and then presenting the operator with a scene in front of his eyes that closely resembles a daytime scene. NVGs have made great advances in recent years in both their power to multiply existing light and their visual acuity. Modern NVGs amplify light in the upper visible (red) spectrum and the just-above-visible (near infrared) spectrum. This area of amplification will be referred to as the “NVG amplification spectrum.” While NVG sensitivity is greatest in the just-above-visible/near infrared region, there is a significant sensitivity in the visible red region as well. Consequently, there is an overlap between the light frequencies that the unaided human eye sees and those light frequencies that NVGs respond to, e.g., the NVG amplification spectrum.
Most modern NVGs have a built-in filter that limits this overlap by making a cleaner NVG cutoff at the lower end of the NVG amplification spectrum. These NVGs are not sensitive to blue and green colored light. NVGs may also employ an automatic brightness control feature, which acts to decrease light amplification as input light levels increase. Automatic brightness control was designed to maintain a constant NVG scene brightness. Brightness control is initiated above a certain input light level threshold and reduces the light sensitivity of the goggles causing a corresponding decrease in system gain, and an overall loss of NVG-aided visual acuity. Higher levels of input light in the NVG amplification spectrum can reduce NVG performance and thus reduce the operator's ability to see a low light level nighttime scene.
Aircraft external lighting have previously been provided by “strobe” lights or incandescent lamps. An unfiltered incandescent bulb emits strongly in the near infrared spectrum. Strobe lights also produce a broad spectrum of light having significant components in the NVG amplification spectrum. Incandescent and strobe lamps also suffer from relatively short service life.
With advances in the intensity of light output from light emitting diodes (LEDs), it is now possible to replace incandescent and strobe lamps with LED light sources. LED light sources are attractive because of their extremely long service life and relatively low power consumption. Several high flux LEDs such as the Luxeon™ emitter from LUMILEDS™ of San Jose, Calif., in certain configurations, can achieve the required light output and radiation pattern for an aircraft position light. Recently developed white LEDs make it possible to produce a chromatically white LED aircraft position light meeting the requirements for intensity and radiation pattern. However, white LEDs emit significant light in the NVG amplification spectrum, and in particular display a spike in light emission at a wavelength of approximately 610 nm, very close to the red portion of the spectrum. As a result, aircraft position lights employing white LEDs are unacceptably bright in the NVG amplification spectrum.
Aircraft flying in civilian airspace are required to turn on their navigation lights. Navigation lights employing incandescent or LED light sources impair the effectiveness of aircrews equipped with NVGs due to the intensity of light emitted in the NVG amplification spectrum. FAA regulations requiring external aircraft lighting have been modified to permit military aircraft operation in military airspace without external lighting in recognition of this problem. It is possible to equip such navigation lights with filters that reduce the light output in the NVG amplification spectrum. However, these filters usually reduce the visible light output below the intensity specified by the relevant regulations. The filters also tend to shift the color chromaticity coordinates outside those permitted by the relevant regulations. In extreme cases it is known to turn off external lights, placing aircraft, aircrews and the public at risk.
There is a need in the art for an aircraft navigation light that provides the required chromaticity, light intensity and radiation pattern for a white position light and emits little or no light in the red and near infrared light spectrum.